1. Field of the Invention
This invention relates to an improvement in the process for preparation of alkoxysilanes by the esterification of chlorosilanes with alcohols. More especially, this invention relates to the preparation of higher yields of alkoxysilane by the esterification of chlorosilanes with alcohols wherein the process is carried out in the presence of a chlorohydrocarbon and in the absence of an acid binding agent.
2. Discussion of the Prior Art
Esterification of chlorosilanes with alcohols en route to the preparation of alkoxysilanes is known and takes place in accordance with the following equation: EQU R.sub.m SiCl.sub.4-m +nR'OH.fwdarw.R.sub.m Si(OR').sub.n Cl.sub.4-m-n +nHCl.
In this equation, R' represents an alkyl radical of 1 to 11 carbon atoms, m is a value of between 0 and 3, n is a value of between 1 and 4 and R represents hydrogen or a C.sub.1 -C.sub.11 alkyl radical.
Difficulties have been encountered in the practical performance in that the hydrogen chloride that is formed in large amounts due to the process, not only cleaves the alkoxy group to alcohol and chlorosilanes but also, especially in the presence of alkanol, cleaves the hydrogen silane bond to yield hydrogen and form an alkoxysilane chlorosilane bond. Furthermore, the hydrogen chloride forms chloroalkanes with the alkanols employed, and intermediately forms water. This water, in turn, tends to hydrolyze the chlorosilanes and alkoxysilanes into hydrolyzates. Unless certain process conditions are formed, the desired ester is usually entirely lost.
As a consequence thereof, a number of attempts have been made to prepare such compounds in a more economical manner. The weakness of the formation of condensate, which originally encumbered the batch processes, due to the aforementioned secondary reactions of HCl with alcohols in the esterification can be largely avoided by the use of modern batch methods. However, these modern methods are limited to the application of those methods which can be performed on a large, commercial scale, inasmuch as systems must be provided to control the large amounts of HCl realized, especially in conjunction with the low boiling temperatures of the starting substances employed and the rapid and severe temperature gradients which are required, both in the reaction chamber and in the exhaust gas, if the reaction is to be conducted safely.
Continuous processes have also been proposed wherein chlorosilanes are esterified in the liquid phase in a reactor equipped with an overflow borrowed from the simple batch technology art. Continuous processes have also been proposed employing a plurality of reactors connected in series in a somewhat countercurrent principle. This type of process, however, has the disadvantage that the hydrogen chloride is removed too slowly and incompletely. This results in the re-cleavage of already formed ester groups and in the commencement of secondary reactions between the alcohols and the hydrogen chloride, with the undesired formation of hydrolyzates.
Another proposed esterification procedure of the chlorosilanes with alcohols has involved the use of a gas phase and temperatures which are above the boiling points of all substances involved, i.e., above the boiling points of both the starting products and the desired end products. This process has, however, the decided disadvantage in that elevated temperatures are required which cause the hydrogen chloride present in the system to produce a particularly rapid promotion of the known secondary reactions. This catalytic effect upon the secondary reactions effects a re-cleavage, alcohol dehydration and the formation of the hydrolyzate.
The particular weaknesses of all of the continuous esterification processes described above lies in the fact that excessively low and incomplete separation of the hydrogen chloride from the reaction mixture is involved. It has already been proposed, therefore, to purge out the HCl forming as a result of the process by passing inert gases, such as nitrogen, over the surface of or through the reaction mixture with the aid, in some cases, of a falling film evaporator. Where such evaporator is employed, an upper temperature limit must not be exceeded. This general procedure, however, has the considerable disadvantage in that the exhaust gas volume, which contains HCl, is enormously increased and vaporization losses ensue. The loss of the material through vaporization losses is determined by the partial pressure of the products. These losses become unreasonably high and virtually preclude the reuse of hydrogen chloride given off as a result of the process.
Furthermore, in the foregoing procedure, powerful cooling apparatus are necessary to reduce the product loss in the inert gas stream. Additionally, extremely dry gases are essential to such a procedure, unless otherwise the formation of siloxanes is intensified.
It is further known to increase the rate of withdrawal of HCl and other hydrogen halides from the reaction mixture during the esterification process by the addition of benzolic or benzinic solvents to the halogen silane, in order thereby to diminish the above-described formation of siloxanes. In spite of these measures, appreciable residual acidites remain in the reaction product in these procedures and they must be counteracted by the addition of acid binding substances such as salts. This addition entails the disadvantage of the need for additional procedures and materials, as well as the filtration of the raw ester and the elution of the salts to reduce the yield losses.
The esterification of chlorosilanes with alcohols is known to proceed in the presence of chlorinated hydrocarbon, wherein the introduction of tertiary alcoholic components in the presence of amines is performed. In this procedure, however, large amounts of salts are also produced which must be removed by additional processing steps.
It therefore has become desirable to provide a simple and effective means for realizing high yields of desired esterification product of chlorosilanes with alkanols. More especially, it has become desirable to provide a process in which efficient removal of HCl formed during the process is ensured thereby precluding the HCl from entering into secondary reactions. More especially, it has become desirable to provide a process for preparing higher yields of desired alkoxysilane by the reduction of secondary reactions.